Lenslet array system incorporating a field lenslet array

ABSTRACT

A lenslet array system for imaging an associated object onto a final image plane includes (i) a first lens assembly including a field limiting mask and a first lenslet array having an associated image plane, (ii) a second assembly including a rear lenslet array, and (iii) a middle lenslet array located between the first and said rear lenslet array. The first lenslet array accepts a full field of view subtends by the associated object and forms a plurality of image sections of the associated object on an intermediate image plane. The first lenslet array includes a plurality of positive power lenslets, each of the plurality of lenslets having a focal length f 1  and accepting a unique segment of the full field of view subtended by the associated object. These segments of the full field of view together comprise the full field of view, and each of the lenslets forms one image section corresponding to its segment of the full field of view. The rear lenslet array has a plurality of positive power lenslets. Each of the lenslets of the second lenslet array (a) reimages one of the image sections located at the intermediate image plane and creates an inverted image of the image section on the final image surface, and (b) together with other lenslets of the rear lenslet array creates an image of the associated object.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following commonly assignedapplications:

(1.) U.S. Ser. No. 08,772,590, filed concurrently herewith by Mark M.Meyers for LENSLET ARRAY SYSTEM (Docket 74960SZS);

(2.) U.S. Ser. No. 08,771,592, filed concurrently herewith by Mark M.Meyers for A LENSLET ARRAY SYSTEM INCORPORATING AN INTEGRAL FIELDLENS/REIMAGER LENSLET ARRAY (Docket 74959SZS);

(3.) U.S. Ser. No. 08,771,123, filed concurrently herewith by Mark M.Meyers for A LENSLET ARRAY SYSTEM WITH A BAFFLE STRUCTURE AND A SHUTTER(Docket 74975SZS);

(4.) U.S. Ser. No. 08/652,735, filed May 23, 1996 by Mark M. Meyers forA DIFFRACTIVE/REFRACTIVE LENSLET ARRAY; and

(5.) U.S. Ser. No. 08/663,887, filed Jun. 14, 1996 by Mark M. Meyers forA DIFFRACTIVE/REFRACTIVE LENSLET ARRAY INCORPORATING A SECOND ASPHERICSURFACE.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to compact, short focal length lenslet arraysystems incorporating a plurality of lenslet arrays. Although notlimited thereto, it is particularly suitable for use with photographicor digital cameras, as well as digital camcoders.

2. Description of the Prior Art

Optical systems using dual microlens arrays (also referred to as lensletarrays) for imaging purposed are known. Such optical systems aredescribed in U.S. Pat. No. 4,632,522. This patent discloses a firstlenslet array imaging a nearby object on an intermediate image plane ina series of image sections and a second lenslet array reimaging theseimage sections onto a final image plane. Because the rays A (FIG. 1)forming the edges of the image sections miss the lenslets of the secondlenslet array, the edges of these image sections will appear dim to thesecond lenslet array. When the second lenslet array reimages the imagesections on the final image plane to form a composite final image, thefinal image will suffer from "banding" effect. That is, the final imagewill be composed of periodic dark and light regions or bands.

U.S. Pat. No. 5,418,583 discloses an illumination device that comprisestwo lenslet arrays. The second lenslet array is located in the vicinityof the focal plane of the first lenslet array. It is not a relay lensletarray and thus it cannot and does not provide a reimaging function.

U.S. Pat. No. 4,988,188 also discloses an illumination device thatcomprises two lenslet arrays. The second lenslet array is located in thevicinity of the focal plane of the first lenslet array. It is not arelay lenslet array and thus it cannot and does not provide a reimagingfunction. The disclosed devise also includes an additional condenserlens which is located behind the second lenslet array. The condenserlens functions as a collimator and is located one focal length away fromthe second lenslet array. Since this lens is a collimator, it collimatesincoming light beams and does not reimage images located at theintermediate image plane in the plane of the second lenslet array.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the banding problemassociated with the above described prior art.

It is an object of the present invention to provide a very compact,easily manufacturable, short focal length, wide angle lenslet arraysystem.

According to the present invention, a lenslet array system for imagingan associated object onto a final image plane includes (i) a first lensassembly including a field limiting mask and a first lenslet arrayhaving an associated image plane, (ii) a second assembly including arear lenslet array, and (iii) a middle lenslet array located between thefirst and said rear lenslet array.

The first lenslet array accepts a full field of view subtends by theassociated object and forms a plurality of image sections of theassociated object on an intermediate image plane. The first lensletarray includes a plurality of positive power lenslets, each of theplurality of lenslets having a focal length f, and accepting a uniquesegment of the full field of view subtended by the associated object.These segments of the full field of view together comprise the fullfield of view, and each of the lenslets forms one image sectioncorresponding to its segment of the full field of view.

The rear lenslet array has a plurality of positive power lenslets. Eachof the lenslets of the second lenslet array (a) reimages one of theimage sections located at the intermediate image plane and creates aninverted image of the image section on the final image surface, and (b)together with other lenslets of the rear lenslet array creates an imageof the associated object.

The middle lenslet array accepts light from the first assembly and islocated (a) between the first lenslet array and the rear lenslet arrayand (b) within 1 mm of the intermediate image plane. The middle lensletarray has at least four positive power lenslets. Each of the lenslets ofthe middle lenslet arrays corresponds to a specific image section andbends and directs the light emanating from the edges of this specificimage section towards a corresponding lenslet of the rear lenslet array.

It is an advantage of the present invention that the lenslet arraysystem of the present invention minimizes "banding" effects present inthe prior art systems.

It is also advantage of the present invention that the lenslet arraysystem of the present invention can present more light rays from theedges of the intermediate image sections to the second lenslet array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art optical system;

FIG. 2 schematically illustrates an exemplary lenslet array system ofthe present invention;

FIG. 3 shows schematically a sectional view of a lenslet array system100 of a first embodiment;

FIG. 4 illustrates the positioning of lenslets 130 and 140 of thelenslet array system 100 relative to one another and relative to theintermediate image plane;

FIG. 5 shows a top view of the baffle plate 150 and the lenslets 130 ofthe first lenslet array 110;

FIG. 6 illustrates a side view of the baffle plate 150 and the lensletarray 110;

FIG. 7 illustrates the baffle structure 160;

FIGS. 8A-C illustrates three possible configurations of a shutter plate170;

FIGS. 9A and 9B illustrate the second embodiment of the lenslet arraysystem;

FIG. 10 illustrates a third embodiment of the lenslet array system;

FIG. 11 illustrates the top view of the embodiment shown in FIG. 10;

FIG. 12 illustrates yet another embodiment of the lenslet array system;

FIG. 13 is a top view of lens contours and baffles for the lens array 10of the present invention;

FIG. 14 is a sectioned view taken along the section lines 2-2 of thelens array of FIG. 13;

FIG. 15 illustrates in cross section a central lenslet 12 of the firstlenslet array 10, 110;

FIG. 16 illustrate in cross section a lenslet of the first lenslet array10, 110 at a 16 degree field angle;

FIG. 17, illustrate in cross section a lenslet of the first lensletarray 10, 110 at a 24 degree field angle;

FIGS. 18 and 19 are a front and rear perspective view of a lenslet 12;

FIG. 20 is a sectioned view of the lenslet 12 of FIG. 19;

FIGS. 21A and 21B illustrates an aperture array positioned over alenslet array with the spherical surface of each lenslet defined withtopographical lines with the lenslets physical centers diverging withrespect to centers of the image sections 135 (in FIG. 21A) andconverging with respect to the centers of the image sections 135 (inFIG. 21B);

FIG. 22 illustrates, in a cross section view, a first lenslet array 110having an array of field stops and an array of aperture stops positionedin front of this lenslet array;

FIG. 23 illustrates, in a cross section view, a first lenslet array 110having an array of field stops positioned in front of this lenslet arrayand an array of aperture stops positioned between the lenslet array 110and the intermediate image plane 136;

FIG. 24, illustrates a cross section of the portion of a lenslet 10, 130associated with a 0 degree field angle;

FIG. 25 illustrates a cross section of the portion of a lenslet 10, 130associated with a 14 degree field angle;

FIG. 26 illustrates a cross section of the portion of a lenslet 10, 130associated with a 20 degree field angle; and

FIG. 27 illustrates a cross section of a centrally located lenslet 10,140 used in a reimaging lenslet array.

DESCRIPTION OF THE PREFERRED EMBODIMENT

I. Lenslet array system description

The lenslet array system is capable of accepting a wide field of view(at least ±30 and preferably ±35 or more degrees). However, it may bebeneficial for use in lenslet array systems accepting a ±10° (or larger)field of view. The lenslet array system comprises a plurality of lensletarrays 10. It is preferred that the total length of the lenslet arraysystem is less than 17 mm and preferably less than 12 mm, and morepreferably less than 8 mm and most preferably less than 5 mm. Thisallows for the fabrication of extremely compact photographic and digitalcameras, as well as video camcoders. (The total length of the lensletarray system is measured from the front most lens array surface to thefinal image plane). The fist lenslet array arrays 10 functions as animaging lens and the rear lenslet array functions as a relay lens (alsocalled a reimaging lens). A middle lenslet array is located between thefirst and the rear lenslet array and functions as an field lens. Eachlenslet array comprises a plurality (over 4) of small lens elements 12.These small lens elements are called lenslets. A given lenslet array maycontain tenths, hundreds, thousands or millions of lenslets 12. Lenslets12 with typically have clear aperture diameters of about 1-2millimeters. However, the fewer the number of lenslets on the array, thelarger the clear apertures of these lenslets. Thus, the clear aperturediameters of individual lenslets may exceed 2 mm. If the field of viewof the lenslet array system is very large, these lenslet arrays tend tocontain a large number of lenslets. The larger the number of lenslets inthe array, the shorter can be the lenslet array's focal length. Thus, anarray with a large number of lenslets can fit into a smaller, morecompact package. The lenslets 12 are arranged on a common substrate 13.It is preferred that this substrate be about 0.5 to 2.5 mm thick. It maybe thicker, but additional thickness is undesirable because it makes theoverall lenslet array system less compact. If this substrate is made toothin it may become fragile. Thus, it is most preferred that thesubstrate thickness be 1-2 mm. The specific parameters for the exemplarylenslets are provided in the "Lens component description" of thespecification.

1. First embodiment.

According to the first embodiment, the lenslet array system 100comprises a plurality of lenslet arrays 10 which include a first lensletarray 110 and a second lenslet array 120 which are shown in FIG. 3.

Lenslets 12 of the first lenslet array 110 are referred to as lenslets130. Each lenslet 130 has positive optical power. It has a focal lengthf₁ of 3.0 mm and an F-number F/2. Each lenslet 130 functions as aseparate imaging lens. Each lenslet 130 is shaped to accept a uniquesegment of the field of view and to create an inverted image sectioncorresponding to this segment of the field of view. That is, eachlenslet 130 creates a small, discrete, inverted section 135 of theoverall image. The multiple image sections are not continuous and arenot oriented properly with one another to permit either a direct viewingor film photography (see FIG. 3). The rear lenslet array 120 is neededto reimage the multiple image sections 135 (located at the intermediateimage plane 136) created by the first lenslet array 110 to form asingle, continuous, correctly oriented image I out of these multipleimages sections 135. The lenslets 12 of the rear lenslet array 120 arereferred to herein as lenslets 140. These lenslets 140 have positiveoptical power, a focal length f₂ of 0.5 mm and the F-number F/2. Eachlenslet 140 corresponds to one of the plurality of lenslets 130 on thefirst lenslet array 110. Each lenslet 140 functions as a relay lens.That is, each lenslet 140 of the second lenslet array 120 reimages andreinverts an image section 135 created by its corresponding lenslet 130of the first lenslet array 110. The image I may be formed on anyphotosensitive surface including photographic film, paper or a CCDarray. It may be preferred in some digital camera application that theimage I created on the photosensitive pixels of the sensing array not becontinuous, because these pixels are not located substantially adjacentto one another. In this case the lenslets 140 reimage the plurality of 5"scrambled" image sections 135 into a plurality of the unscrambled finalimage sections that form an overall, correctly oriented, discontinuousimage I of the object O.

The focal lengths f₁ of lenslets 130 are relatively small, preferablysmaller than about 15 millimeters and more preferably smaller than 12millimeters and even more preferably smaller than about 10 millimeters.Most preferably it will be in the 2 to 7 mm range. As stated above, inthis embodiment it is 3 mm. It may be as small as 1 mm or 0.5 mm. Thesmaller is the focal length f₁, the more compact is the total length ofthe lenslet array system. However, the smaller is the is focal length f₁(given specified field of view for each of the lenslet 120), the smalleris the size of an intermediate image section 135. Thus, if f₁ is in therange of 1 mm to 0.5 mm (or smaller) the size of the image sections 135may become to small for their proper reimaging on film. If film is usedas the photosensitive medium, the film grain size 20 will be one of themajor factors determining the smallest possible size for the imagesections 135. Alternatively, in digital camera applications, the size ofthe focal length f₁ may be limited by the size of the photosensitivepixels. Thus, it is preferred that f₁ be greater than 0.5, andpreferably greater than 1 mm.

FIG. 3 illustrates that the first lenslet array 110 forms image sections135 (on an intermediate image plane 136) of an object O which is locatedat an object distance d₁ from the front vertex of the central lenslet130. It is preferred that the object distance d₁ be equal to more than20 times the focal length f₁ of the lenslet 130 (i.e., d₁ is greaterthan 20 times f₁, where f₁ is less than 15 mm). Than, any object whichis located at a typical viewing distance for the camera typeapplications (i.e. 300 mm or more in front of the camera) will belocated at object distances which are effectively at infinity for thefirst lenslet array 110. The intermediate image plane 136 serves as anobject plane for the lenslet array 120 and should be located a distanced₂ behind the focal plane F of the first array, the distance d beingless than 5% of f₁. Therefore, this lenslet array system embodiment isinsensitive to the changes in the object position. This is schematicallyillustrated in FIG. 4, where the distance d₂ shown greatly exaggerated.

It is preferred that in order to provide a reasonable back focusdistance, the ratio f₁ to f₂ be between 1 and 10, where f₁ is the focallength of the front lenslet array 110 and f₂ is the focal length of therear, lenslet array 120. It is even more preferred that this ratio bebetween 1 and 7 and most preferred to be between 1 and 5. The shorter isthe f₂ the more compact is the overall lenslet array system. If theratio of the two focal lengths f₁ /f₂ becomes less than 1, the backfocus distance tend to become too large, and the lenslet array systemtends to become less compact. If the ratio of the two focal lengths f₁/f₂ becomes larger than 20, the field of view of the relay lenslets 140becomes large, the lenslet array system aberrations become difficult tocorrect and the image quality suffers. As stated above, in thisembodiment f₁ is 3.0 mm and f₂ is 0.5 mm. The value of the ratio f₁ /f₂is 6.0. The spacing between the rear and the intermediate image planeshould be greater or equal to about 1.2 times f₂ in order to provideenough space between the shutter and the rear lenslet array 120.

It is noted that if the individual lenslets 130 of the first lensletarray see relatively large fields of view, they form image sections 135which are relatively large in size. Applicant discovered that some ofthe field light rays emerging from the edges of the intermediate imagesegments 135 may miss clear apertures of lenslets 140. In order to avoidthis problem an additional lenslet array 115 may be placed between thefirst lenslet array 110 and the rear lenslet array 120. This additionallenslet array performs the function of the field lens. The lenslet arraysystem with such additional lenslet array is disclosed in more detail inthe second lenslet array system embodiment.

Baffling system associated with the lenslet array system 100.

An opaque baffle plate 150 having multiple openings 155 is positioned infront of the lenslet array 110. (FIG. 5 and 6). It may serve as anaperture stop array. The openings 155 of the plate 150 are aligned withthe individual lenslets 130 to allow the proper light beams to gothrough the lenslet array system. The size and the shape of the lenslets130 match the size and the shapes of the openings 155. The opaqueportion of the plate 150 prevents the light rays from passing betweenthe lens elements, from propagating further into the lenslet arraysystem as unwanted (also referred to as stray) light, and from reducingimage quality. The function of the baffle plate 150 may also beincorporated into the structure of the lenslet array 110 by making thesubstrate of the lenslet array 110 opaque to light in all areas notoccupied by the lenslets 130.

The field of view of the first lenslet array 110 is limited by an opaquebaffle structure 160. (FIG. 7) The baffle structure 160 includes a fieldlimiting opaque plate 162 with a plurality of apertures 165, and aplurality of baffle walls 167. These baffle walls 167 absorb stray lightrays (A) exiting lenslet 130 and thus prevent it from ether propagatingtowards another (inappropriate) image segment 135 or from reflecting offthe walls and propagating towards the image segment associated with thespecific lens element 130. The baffle walls 167 may also serve asspacers between the lenslet array 110 and the intermediate image plane.The opaque plate 162 with a plurality of apertures 165 functions as afield stop. More specifically, the opaque plate 162 is located at ornear the intermediate image plane 136. Each aperture 165 frames a smallimage section 135 created by one of the lenslets 130 and in combinationwith the opaque section of the plate 162 surrounding this apertureserves as a field stop for this lenslet. The opaque section of the plate162 also blocks unwanted field rays B and prevents these rays frompropagating towards the relay lenslet array 120. A shutter 170 islocated in a vicinity of the plate 162. It is composed of a series ofholes 175 in an opaque shutter plate 172. The examples of shutter 170are schematically illustrated in FIG. 8A-8C. The shutter 170 may belocated in front or behind the intermediate image plane. The holes 175of the shutter 170 are aligned with the opaque areas of the fieldlimiting plate 162, but can be displaced, for example by a springoperated mechanism M, to align with the holes 165 (FIG. 7) in order toexpose the photosensitive medium for the desired amount of time. Inorder for this type of shutter mechanism to work, the intermediate imagesections located at or near the focal plane of the imaging lenslets 130must be equal or smaller than 1/2 the spacings d' between (the centersof) lenslets 130. It is noted that a different shutter arrangement mayalso be utilized. For example, conventional shutters such as a focalplane shutter, or an iris shutter located in font of the lenslet arraysystem may be used to prevent the photosensitive area from beingexposed. If a conventional shutter is used, the spacing between theimage sections 135 may be smaller or larger than 1/2 spacing between thecenters of the lenslets.

2. Second embodiment.

Lenslet array system 200 of the second embodiment of the presentinvention is schematically illustrated in FIGS. 9A and 9B. Lenslet arraysystem 200 comprises three lenslets arrays 110, 115, and 120. Lensletarrays 110 and 120 of the second embodiment serve the same function aslenslet arrays of 110 an 120 of the first embodiment. The lenslet array110 is the imaging lenslet array and the lenslet array 120 is the relayarray. Lenslet array 120 comprises a plurality of lenslets 140. TheLenslet array 115 serves the function of the field lens. It is locatedat or near the intermediate image plane and bends the field rays towardsthe optical axis of the individual lenslet, making the lens bundlesincident on each of the lenslets 140 smaller. The field lenslet array115 is especially useful in a lenslet array system with a large field ofview because it sends more light rays towards the lenslet array 120 andthus allows for the smaller size lenslets 140 in the lenslet array 120.In this system, the field plate 162 and the shutter 170 may be locatedeither in front or behind the field lens array 115. FIG. 9A shows thefield lens array 115 located in front of the field plate and theshutter, while FIG. 9B shows the field lens array 115 located in behindof the field plate and the shutter. As stated above, the holes 175 ofthe shutter 170 are aligned with the opaque areas of the field limitingplate 162, but can be displaced, for example by a spring operatedmechanism M, to align with the holes 165 in order to expose thephotosensitive medium for the desired amount of time. In FIG. 9A theshutter is in the closed position, while in FIG. 9B the shutter is shownin the open position. As in the previous embodiment the intermediateimage plane 136 serves as an object plane for the lenslet array 120 andshould be located a distance d₂ behind the focal plane F of the firstarray, the distance d₂ being greater than 5% of f₁. Therefore, thislenslet array system embodiment is insensitive to the changes in theobject position.

3. Third embodiment.

Lenslet array system 300 of the third embodiment of the presentinvention is schematically illustrated in FIG. 10. Lenslet array system300 comprises two lenslets arrays 110, and 120. Lenslet arrays 110 and120 of the lenslet array system 300 serve the same function as lensletarrays of 110 an 120 of the first embodiment. More specifically, thelenslet array 110 is the imaging lenslet array. It is made of aplurality of lenslets 130. In the lenslet array system 300 all of thelenslets 130, with an exception of a center lenslet are decenteredlenslets. That is, their individual optical axis 18 and their unit cellaxis of symmetry 14 do not overlap. This is illustrated in FIG. 11. Theunit cell axis of symmetry is defined as an axis of symmetry of thespace occupied by an individual lenslet. The parameters for an exemplarylenslet 110 are described in detail in the "Lens component description"of the specification. This arrangement of lenslets on the lenslet array110 allows each of the individual image segments 135 to be centered onthe unit cell axis 14 of corresponding lenslet 130. This allows thecorresponding lenslet 140 of the second lenslet array 120 to be locatedright behind its corresponding lenslet 130. The optical axis 18' of thesecond lenslet array 120 may then be collinear with the unit cell axis14 of the first lenslet array 110. This allows for a very compactlenslet array system.

As stated above, the lenslet array 120 is the relay (or reimager)lenslet array. The lenslet array 120 reimages the multiple, invertedimage segments 135 located at the intermediate image plane into acontinuous, correctly oriented image located at the final image plane.In addition, the lenslet array 120 serves the function of the fieldlens. That is, lenslet array 120 of this embodiment is a field lensarray. More specifically, the front (object facing) surface S₃ oflenslet 140 is located at or in the vicinity of the intermediate imageplane 136 and bends the field rays C towards the optical axis 18' ofthis lenslet. It is preferred for the surface S₃ to be spaced slightlyaway from the intermediate image plane so that the dust particles orscratches present on the surface S₃ would not be reimaged onto the finalimage plane. The rear surface S₄ (i.e., the surface facing the finalimage plane) of the lenslet 140 provides the re-imaging function. Thefinal size of the image is about the same as the size of the lensletarray 120. The baffling system employed in lenslet array system 300 issimilar to the one employed by the lenslet array system 100 and 200 andis also shown schematically in FIG. 10. It includes an opaque bafflelayer 150 with multiple transmissive sections or apertures 155, and abaffling structure 160 which is similar to the baffling structure 160 ofthe lenslet array system 100. A shutter, similar to the shutter 170, islocated behind the baffling plate 162. In order for the shutter to work,the size of the image sections 135 located at the intermediate imageplane 136 must be smaller than 1/2 spacing d' between lenslet unit cellcenters. However, if a different shutter arrangement is utilized, thesize of the image sections 135 may be larger.

4. Fourth embodiment

Lenslet array system 400 of the third embodiment of the presentinvention is schematically illustrated in FIG. 12. Lenslet array system400 comprises two lenslets arrays 110, and 120. Lenslet arrays 110 and120 serve the same function as lenslet arrays of 110 an 120 of the thirdembodiment. More specifically, the lenslet array 110 is the imaginglenslet array. It is made of a plurality of lenslets 130. All of thelenslets 130, with an exception of a center lenslet are "tilted"lenslets. That is, the central ray in their field of view is notperpendicular to the intermediate image plane 136. This arrangement isdescribed in detail in the "Lens component description" of thespecification. The lenslet array 120 is the relay (or reimager) lensletarray and comprises a plurality of lenslets 140. In addition, thelenslet array 120 is also designed to serve as a field lens array. Thus,the front surfaces of lenslets 140 are convex. The front (object facing)surface of the lenslet array 120 is located at or in the vicinity of theintermediate image plane and bends the field rays towards the opticalaxis 18' of the lenslet 140 of the second lenslet array 120. The rearlenslet surface S₄ (i.e., the surface facing the final image plane) ofthe lenslet array 120 serves the re-imaging function. The lenslet array120 reimages the multiple, inverted image segments 135 located at theintermediate image plane into a correctly oriented image located at thefinal image plane. This image is continuous if the lenslet array systemimages on film. It may be continuous or discontinuous if a CCD or asimilar array is used as a photosensitive surface.

As can be seen, the centers of lenslets 130 are displaced relative tothe centers of the corresponding lenslets 140. The lenslet array system400 is not as compact as lenslet array system 300. This is because inorder to achieve the same final image size, as achieved by the lensletarray system 300, the lenslet array 110 of the lenslet array system 400needs to be larger than the lenslet array 120.

II. Lens component description

1. Imaging lenslet array.

EXAMPLE 1

Referring to FIG. 13, a lenslet array 10 is formed with an array ofachromatized refractive/diffractive lenslets 12 or refractive lenslets.Such an array may be used as first array 110 in the first, the second orthe third lenslet array system embodiment, respectively. Theseembodiments are described in the "lenslet array system description"section of this application. To be observed in this figure is that thecenter of the optical axis 18 of each lenslet 12 is displaced by adistance d relative to the fixed unit cell to unit cell distance X as afunction of its radial distance from the optical axis of the centrallenslet. The lines 15 appearing around the optical axis 18 of eachlenslet 12 are topographical lines indicating changes in height of thelenslet's surface. An opaque mask 16 fills the areas between thelenslets 12 to prevent stray light from propagating farther into theoptical system. The array depicted in FIG. 13 represents only a smallportion of an array that will be used in an actual camera. In an actualimplementation many more lenslets are used to form the array. Otherconfigurations of the lenslets 12 may be used such as forming the outerperiphery of each lenslet as a square, hexagon, or circle, withoutdetracting from the invention.

In order for the lenslet array 10 to see different fields of view theoptical axis 18 of the lenslets 12 in the lenslet array are located at adistance which becomes progressively larger than the center-to-centerdistance of the unit sells of the lenslet array. As stated above, thedisplacement d of the lenslets optical axis 18 increases radially fromthe center of the array. Decentering a lenslet tends to bend rays fromoff-axis field angles to be incident perpendicular to the intermediateimage plane. By moving the optical axis 18 of the lenslet further outradially with increasing distance from the center of the array, theangular location of object at the center of the field of view for agiven lenslet originates from increasingly off-axis segments of thetotal field of view (see FIG. 14).

For instance, the required decenter for a lenslet 12 of focal lengthFL_(i) necessary to deflect the ray from the desired field angle intothe center of the array element's field stop can be determined from theparaxial ray tracing equations. The paraxial equations are

    y'=y.sub.0 +nu(t/n)

    n'u'=n.sub.0 u.sub.0 -yφ

where

y'=height after propagation to next surface

y₀ =height at previous surface

u=paraxial slope angle (radians)

u₀ =slope angle before refraction

φ=power of array element (φ=1/FL_(i))

n=refractive index of the medium

Therefore, the displacement d for a given lenslet with powerφ=(1/FL_(i)) that is necessary to bend the central ray from a givenangle of incidence u₀ to a desired angle u', after refraction is givenby

    d=y=(n.sub.0 u.sub.0 -n'u')/FL.sub.i

The invention utilizes an array of lenslets where the local displacementof the lenslets optical axis varies as a function of radial positionrelative to the center of the lenslet array system's optical axis, sothat, to first order

    d(r)=(n.sub.0 u.sub.0 (r)-n'u'(r))/FL.sub.i

The invention consists of adjusting the lenslet decenters so thatu'(r)=0 for the central ray within a given lenslet's field of view (seeFIG. 14). In this case the decenter d necessary for a given element isapproximately a linear function of the element's radial distance fromthe system's optical axis.

Referring now to FIG. 14, the lenslet array 10 is positioned over anintermediate image plane and creates a plurality of image segments 135.A shutter may be located next to the intermediate image plane 136. Thelens array 10 is maintained a distance apart from the shutter by spacers22 that may also serve the function of being baffles. As can be seenfrom FIG. 14, the opaque baffles 16 on the lenslet array 10 may becombined with a field stop (aperture plate 40) to limit the field ofview of any particular photosensor so that it does not overlap the fieldof view of it neighbors by a large amount. The aperture plate 40 ispreferably positioned approximately 0.5 mm to 2 mm from the surface ofthe lenslet array 10. To further define a field of view seen by eachindividual lenslet 12 the aperture plate 40 may be a layer of clearglass having a dyed photoresist mask pattern formed on one of itssurfaces. The center of the apertures in the aperture plate 40 arealigned to the center of the field of view (CFDV) of a correspondinglenslet. The spacing of the centers of apertures in the plate 40increases as a function of each lenslet's radial position radially fromthe center of the array causing the aperture plate to be slightly largerthan the associated lens array. The combination of the opaque areas 16with the aperture plate 40 and/or the field limiting plate 162, and agiven lenslet focal length determines the field of view for a particularlenslet on the lenslet array. The lenslet array 10 can be formed byetching a photosensitive pattern into quartz, or formed as an epoxyreplica on a quartz substrate or as a photoresist surface relief part ona glass substrate, or be injection molded as a plastic part.

The lenslets 12, combined with the appropriate field stop (such as theaperture plate 40), and/or the field limiting opaque plate 162, formimages of a small segment of the field of view on an intermediate imageplane 136. By forming the lenslets 12 with decentrations d of theoptical axis 18 which increase radially across the lenslet array, theangle which the ray beam incident on any of the lenslet 12 will increaseas a function of radial position of this lenslet on the array.Therefore, by appropriately adjusting the decenters of each lenslet eachimage section corresponds to a unique or different segment of the scene.

Lenslet array 10 may an array of aspheric lenslets to improve thelenslet array system performance. However, even aspheric lenslets do notcorrect for the variation in focal length as a function of wavelength ifthese lenslets are formed from a single refractive material. That is,the spot sizes at the image plane will vary as a function of color. Inorder to correct the chromatic aberration, an improved lenslet arraysystem including an array of diffractive/refractive hybrid lenslets maybe used instead of an array of purely refractive lenslets. The imagingproperties of diffractive optics are strongly wavelength dependent. Whenmodeling a diffractive optic this phenomena can be represented as adirect dependence of equivalent refractive index n(λ) on wavelength(λ):

    n(λ)= λ.sub.C /λ!(n.sub.C -1)-1

Diffractive elements impart all of their wavefront bending in anextremely thin layer. This is accounted for in the Sweat model bymodeling the diffractive as a very high index material (n_(C) =10,000)with very low curvatures (weak surfaces) on each surface. Thecorresponding focal length f(λ) can then be determined from:

    f(λ)= n(λ)-1!Δc

    so that

    f(λ)=(λ.sub.C /λ)f.sub.C

where λ_(C) =design center wavelength

The resultant dispersion V_(diff) of the diffractive element is:##EQU1## which reduces to: ##EQU2##

For designs using: λ_(C) 587 nm, λ_(S) =486 nm, and λ_(L) =656 nm thevalue for ν_(diff) is -3.5.

For other wavelength bands of interest an appropriate ν_(diff), andelement power distribution can be calculated. The direct dependence ofthe equivalent refractive index on wavelength leads to a small,negative, ν_(diff) and a high level of wavelength dispersion associatedwith a first order (m=1) diffractive optical element.

Due to the variation of refractive index with wavelength, a singleelement lens has a variation of focal length with wavelength. Twomaterials with different dispersions can be used to form a doublet lenswhich has the same focal length at two wavelengths and reduced variationover the whole spectrum. The relative distribution of focal powersrequired to achieve this is given by ##EQU3##

The negative ν_(diff) of the diffractive surface allows forachromatization of single element hybrid refractive/diffractive lensesutilizing a positive focal length diffractive and refractive component.It also decreases the focal length and F/# required of the doubletcomponent lenses because a refractive doublet consists of a positivecrown (low dispersion) lens with a shorter focal length and smaller F/#than is required for a singlet and a negative flint element whichincreases the doublet focal length to the correct value and compensatesfor the dispersion of the positive lens. This effect also decreases thesize and weight of the positive element of a hybridrefractive/diffractive element.

For traditional achromatization in the visible regime (d-e-f lines) withPMMA (polymethylmethacrylate) plastic lenses the fraction of power inthe refractive and diffractive portions would be

    .sup.Φ ref/.sup.Φ total=94.25%

    .sup.Φ diff/.sup.Φ total=5.75%

It is obvious to a person skilled in the art that one can utilizesubstrates such a quartz, or optical glasses such as BK7, or utilizeepoxy replicas on glass substrates.

The use of diffractive/refractive hybrid achromatization enables the useof diffractive surfaces with longer focal lengths and higher F-numbers.The high F-number makes the diffractive element easier to manufacturedue to the large zone spacings. For instance, for a lenslet with aF/#=3.0 mm and a F/#=2.0 the first order analysis of the refractive anddiffractive portions would provide focal lengths (f) and F numbers (F#)of

    .sup.f ref=3.186 mm F/#=2.12

    .sup.f diff=52.176 mm F/#=34.8

This assumes that the diffractive portion of the lenslet was used purelyfor first order chromatic correction.

The diffractive lenslets 12 are shown in FIGS. 15, 16, and 17, for theon-axis, 16 degrees, and full field of view. To be noted in a comparisonof these figures is that the optical axis 18 is moving radially outwardsas a function of field angle. It is noted that the opaque baffle 16 actsas the aperture stop for the system and limits the field of view of agiven lenslet 12.

The following table lists the optical design parameters for three arrayelements at 0, 16, and 24 degrees. The array elements at theintermediate field angles can be interpolated from these values.

EXAMPLE 1.A

0 Degree Lenslets

EFL=3.0 mm

F/#=2.0

                  TABLE 1                                                         ______________________________________                                        Surface No.                                                                             Radius       Thickness                                                                              Glass                                         ______________________________________                                        Object                 Infinity Air                                           1 Aperture Stop        2.0      BK7                                           2         -1.639       3.0      Air                                           Aspheric Coeff.                                                                         k = 0.973    Diffractive                                                                            DF3 = -8.2 × 10.sup.-3                            AD = 1.04 × 10.sup.-1                                                                Coeff.   DF5 = -8.4 × 10.sup.-3                            AB = -3.35 × 10.sup.-2                                                                        DF10 = -8.3 × 10.sup.-5                           AF = 1.17 × 10.sup.-1                                                                         DF14 = 1.4 × 10.sup.-3                            AG = -7.75 × 10.sup.-2                                                               Element  DF21 = 4.5 × 10.sup.-4                                         Decenter DF27 = -2.3 × 10.sup.-3                                        DC = 0.0                                               Image                           Air                                           ______________________________________                                    

EXAMPLE 1.B

16 Degree Lens

EFL=3.0 mm

F/#=2.0

                  TABLE 2                                                         ______________________________________                                        Surface No.                                                                             Radius      Thickness Glass                                         ______________________________________                                        Object                Infinity  Air                                           1 Aperture Stop       2.0       BK7                                           2         -1.602      3.0       Air                                           2 Aspheric Coeff.                                                                       k = -0.670  Diffractive                                                                             DF3 = -4.8 × 10.sup.-4                            AD = -7.0 × 10.sup.-3                                                               Coeff.    DF5 = -4.7 × 10.sup.-3                            AE = 2.1 × 10.sup.-2                                                                          DF10 = 1.4 × 10.sup.-2                            AF = -5.9 × 10.sup.-3                                                                         DF14 = -3.6 × 10.sup.-3                           AG = 6.5 ×10.sup.-4                                                                           DF21 = -5.2 × 10.sup.-3                                       Element   DF27 = 6.2 × 10.sup.-4                                        Decenter                                                                      DC = -0.513                                             Image                           Air                                           ______________________________________                                    

EXAMPLE 1.C

24 Degree Lens

EFL=3.0 mm

F/#=2.0

                  TABLE 3                                                         ______________________________________                                        Surface No.                                                                             Radius      Thickness Glass                                         ______________________________________                                        Object                Infinity  Air                                           1 Aperture Stop       0.5       BK7                                           2         -1.509      0.391     Air                                           Aspheric Coeff.                                                                         k = -0.570  Diffractive                                                                             DF3 = 7.8 × 10.sup.-3                             AD = 0.0    Coeff.    DF5 = 5.2 × 10.sup.-3                             AE = -4.6 × 10.sup.-3                                                                         DF10 = 1.0 × 10.sup.-2                            AF = 9.5 × 10.sup.-3                                                                          DF14 = -6.3 × 10.sup.-2                           AG =-1.2 × 10.sup.-3                                                                          DF21 = -2.9 × 10.sup.-3                           AH = 1.4 × 10.sup.-4                                                                Element   DF27 = 7.3 × 10.sup.-4                                        Decenter                                                                      DC = -0.872                                             Image                           Air                                           ______________________________________                                    

where the aspheric surface profile is defined by ##EQU4## and thediffractive phase profile is defined as ##EQU5## where λ₀ =546.1 nm.

EXAMPLE 2

Referring to FIGS. 18 and 19, the first lenslet 12, 130 is one elementof a first lenslet array 110 that is formed with a number of lenslets.Such an array may be used as a first lenslet array in the first, secondor fourth lenslet array system embodiments, respectively. Each lenslet(12, 130) is formed with a diffractive pattern S' formed on arefractive, spherical surface S₁. Opposite the surface S₁ is a secondsurface S₂ Surface S₂ is an aspheric surface. The faceted surface of thediffractive pattern S' is seen more clearly in the cross-section of FIG.20. At a radius R₁ the second surface S₂ is convex and transforms to aconcave surface at the radius R₂ (where R₂ is larger than R₁). Thesecond surface S₂ is defined by a polynomial asphere which exhibits aninflection at the radius R₃ (where R₃ is larger than R₂). Both the firstand the second surfaces are substantially perpendicular to the opticalaxis 18 of the lenslet 12. The lenslet 12 may be formed as an epoxyreplica via use of a quartz or a photoresist mask on a glass substrate,or be injection molded as a plastic part.

Referring to FIG. 20, incident light rays 30 pass through an aperturestop array 40 and are focused onto a focal plane (F, 50) by the lenslet12, 130. The diffractive/refractive surface comprised of S₁ and S'corrects the chromatic aberrations and provides the majority of thefocusing power while the aspheric surface S₂ provides for correction offield dependent aberrations such as petzval curvature, astigmatism, andcoma. The lens has an F# of 2.0 and a FL of 3.0.

Referring now to FIGS. 21A and 21B, The individual lenslets 12 of FIGS.20 and 21 are segmented and formed into a lenslet array 10. To beobserved in FIGS. 21A and 21B is that the center of the optical axis 18of each lenslet 12 is displaced outward as a function of its radialdistance from the optical axis of the central lenslet while in FIG. 21B,the optical axes 18 are displaced inward. The lines 15 appearing aroundthe optical axis 18 of each lenslet 12 are topographical lines generallyindicating changes in height of the lenslet's surface. An array ofopaque baffles 16 also serving as an aperture stop fills the areasbetween the lenslets 12 to prevent stray light from reaching furtherinto the lenslet array system. The array depicted in FIGS. 21A and 21Brepresent only a small portion of an array that would be used in anactual camera. The optical axis of each lenslet, and in turn the lensletitself is not aligned directly over its corresponding image section.Instead the lenslets are displaced so as to form image sections 135 atregularly spaced intervals on the intermediate image plane 136. Otherconfigurations of the lenslets may be used such as forming the outerperiphery of each lenslet as a square, hexagon, or circle, withoutdetracting from the invention.

The reason that the invention uses only portions of the lenslets is thatonly a fraction of the lenslet is used for a particular angular field ofview for an associated image segment pixel.

FIG. 22 and 23 show cross-sections taken along the section line 4-4 inFIGS. 21A and 21B, illustrating the lenslet array 10, 110 positionedover the intermediate image plan and forming a number of image sections135 corresponding in number to at least the number of lenslets formingthe array 10, 110. The lenslet array 10, 110 is maintained a distanceapart from the intermediate image plane by spacers 167 that also servethe function of being baffle walls. The aperture stop array 40 incombination with the baffle walls 167 and the field stop array 42 limitthe field of view of any particular photosensor so that it does notoverlap the field of view of it neighbors by a large amount. The fieldstop array 42 is positioned approximately 0.5 mm to 2 mm from thesurface of the lenslet array 10. The center of the apertures in theaperture stop array and field stop array, 40 and 42, respectively arealigned to the center of the field of view of a corresponding lenslet.The spacing of the centers increases as a function of each lenslet'sfield angle from the center of the array causing the aperture stop arrayto be slightly larger than the associated lenslet array. The combinationof the aperture stop array 40 with the field stop array 42 and a givenlenslet focal length determines the field of each of the lenslets andthe position of image sections on the intermediate image plane.

As stated above, an improved lenslet arrays including adiffractive/refractive hybrid lenslets is used to correct the chromaticaberration present due to use of a single refractive material. Thediffractive lenslets 12 are shown in FIGS. 24, 25, and 26, for theon-axis, 14 degrees, and full field of view. To be noted in a comparisonof these figures is that the optical axis 18 is moving radially outwardsas a function of field angle while the unit cell 14 is incident normalto the plane of the intermediate image plane 136. The opaque mask 16acts as the aperture stop for the system as well as limiting the fieldof view of a given lenslet.

The following table lists the optical design parameters for centrallylocated lenslet.

EFL=0.5 mm

F/#=2.0

                  TABLE 4                                                         ______________________________________                                        Surface No.                                                                             Radius     Thickness Glass                                          ______________________________________                                        Object               Infinity  Air                                            1 Aperture Stop                                                                         Infinity   0.9083    Air                                            2         2.805      2.9999    BK7                                                                 Diffractive                                                                             DF1 = 1.058410.sup.-2                                               Coefficients                                                                            DF2 = 9.572 × 10.sup.-4                  3         -2.417     1.704     Air                                            Aspheric  AD =                                                                Coefficients                                                                            0.3245 × 10.sup.-1                                                      AE =                                                                          0.4534 × 10.sup.-2                                            Image                          Air                                            ______________________________________                                    

As stated above, the aspheric surface profile is defined by equation (1)##EQU6## The diffractive phase profile rotationally symmetric and isdefined as ##EQU7## where λ₀ =546.1 nm

2. The Relay lenslet array

Referring to FIG. 3 and 4 a relay lenslet array 10 is formed with anarray of refractive lenslets 12. Such an array may be used as second(reimaging) lenslet array 120. In this example the lenslets are formedof glass. They have an effective focal length if 0.5 mm. The followingtable lists the optical design parameters for a centrally locatedlenslet comprising array 120. This lenslet is illustrated in FIG. 27.

                  TABLE 5                                                         ______________________________________                                        Surface No. Radius         Thickness Glass                                    ______________________________________                                        Object                     0.19      Air                                      1           infinity       0.8       BK7                                      2 Aperture Stop                                                                           -0.25936*      1.548     AIR                                      Aspheric Coeff.                                                                           k = 0.0                                                                       AD = -0.332E + 01                                                             AE = 0.833E + 03                                                              AF = -0.216E + 05                                                             AG = 0.67373E + 05                                                Image                                Air                                      ______________________________________                                         *Aspheric surface. The aspheric surface profile is defined by equation 1      listed above.                                                            

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A lenslet array system for imaging an associatedobject onto a final image surface, said lenslet array systemcomprising:(1) a first lenslet array having (a) an associated focalplane, (b) accepting a full field of view subtended by the associatedobject, (c) forming a plurality of image sections of the associatedobject at an intermediate image plane, (d) including a plurality ofpositive power lenslets, each of said plurality of lenslets(i) having afocal length f₁, (ii) accepting a unique segment of the full field ofview subtended by the associated object where these segments of the fullfield of view together comprise the full field of view, and (iii)forming one image section corresponding to its segment of the full fieldof view; (2) a second assembly including a rear lenslet array, said rearlenslet array having a plurality of positive power lenslets, each ofsaid positive power lenslets of said rear lenslet array (a) reimaging anassociated one of said image sections located at said intermediate imageplane, (b) creating an inverted image of said associated image sectionon the final image surface, and (c) together with other lenslets of saidrear lenslet array creating an image of the associated object; and (3) amiddle lenslet array located (a) between said first lenslet array andsaid rear lenslet array and (b) within 1 mm of said intermediate imageplane, and accepting light from said first assembly, said middle lensletarray having at least four positive power lenslets, each of saidlenslets corresponding to a specific image section and bending anddirecting the light emanating from the edges of said specific imagesection towards a corresponding lenslet of said rear lenslet array.
 2. Alenslet array system according to claim 1 wherein said intermediateimage plane is substantially coplanar with the focal plane associatedwith said first lenslet array.
 3. A lenslet array system according toclaim 2 wherein at least one surface of said lenslets of said middlelenslet array and at least one surface of said lenslets of said rearlenslet array is convex.
 4. A lenslet array system according to claim 2wherein at least one surface of said lenslets of said middle lensletarray and at least one surface of said lenslets of said rear lensletarray is convex and wherein said middle lenslet array is located within0.5 mm of said intermediate image plane.
 5. A lenslet array system forimaging an associated object onto a final image surface, said lensletarray system comprising:(i) a first assembly having a field-limitingmask and a first lenslet array having an associated focal plane, saidlenslet array forming a plurality of image sections of the associatedobject at an intermediate image plane, said first lenslet arrayincluding a plurality of positive power lenslets, each of said pluralityof lenslets having a focal length f₁ and accepting a unique segment of afull field of view subtended by the associated object, where thesesegments of the full field of view together comprise the full field ofview, and each of said lenslets forming one image section correspondingto its segment of the full view; (ii) a second lenslet array locatedwithin 1 mm of said intermediate image plane and accepting light fromsaid first assembly, said second lenslet array having a plurality ofpositive power lenslets; and (iii) a third lenslet array accepting lightfrom said second lenslet array, said third lenslet array having aplurality of positive power lenslets, each of said positive powerlenslets of said third lenslet array (a) reimaging one of said imagesections located at said intermediate image plane and creating aninverted image of said image section on the final image surface, and (c)together with other lenslets of said third lenslet array creating animage of the associated object.
 6. A lenslet array system according toclaim 5 wherein said intermediate image plane is substantially coplanarwith the focal plane associated with said first lenslet array.
 7. Alenslet array system according to claim 6 wherein at least one surfaceof said lenslets of said second lenslet array and at least one surfaceof said lenslets of said third lenslet array is convex.
 8. A lensletarray system for imaging an associated object onto a final imagesurface, said lenslet array system comprising:(i) a first assemblyhaving a field-limiting mask and a first lenslet array having anassociated focal plane, said lenslet array forming a plurality of imagesections of the associated object at an intermediate image plane, saidfirst lenslet array including a plurality of positive power lenslets,each of said plurality of lenslets having a focal length f₁ andaccepting a unique segment of a full field of view subtended by theassociated object, where these segments of the full field of viewtogether comprise the full field of view, and each of said lensletsforming one image section corresponding to its segment of the full view;(ii) a second lenslet array located within 1 mm of said intermediateimage plane and accepting light from said first assembly, said secondlenslet array having a plurality of positive power lenslets; and (iii) alens unit accepting light from said second lenslet array, said lens unit(a) having a positive optical power, (b) reimaging said image sectionslocated at said intermediate image plane and creating an inverted imageof said image sections on the final image surface, thereby creating asingle continuous image of the associated object.
 9. A lenslet arraysystem according to claim 8 wherein said lens unit comprises a lensletarray of positive power lenslets.